This invention relates to recovery of dissolved molybdenum from spent acid solutions and, more particularly, to recovery of dissolved molybdenum from a spent solution of mixed sulfuric acid-nitric acid, with the recovered molybdenum being in a high purity form suitable for use.
In the processing of tungsten filament coils and especially so-caled coiled coils, a very fine tungsten wire is wrapped about a small molybdenum mandrel to form a first coil. The coiled tungsten and molybdenum mandrel are then wrapped about a second mandrel so that the tungsten is in the form of a coil which in turn is formed into a coil. The formed coiled-coil is then slipped off of the largest mandrel, leaving the first molybdenum mandrel in place within the turns of the smaller tungsten coil. After cutting to size, this member is immersed in a solution of mixed sulfuric acid and nitric acid which dissolves the retained molybdenum mandrel leaving the coiled-coil tungsten intact. The resulting coiled-coil of tungsten is then in a form suitable fo use, such as a filament for an electric lamp. A molybdenum mandrel is also used in the manufacture of single coil lamp filaments.
The relative concentrations of the mixed sulfuric acid and nitric acid are subject to some variation and a mixed acid solution of sulfuric acid (normality of 13) and nitric acid (normality of 7) has been found to be very suitable. When this acid solution has sufficient molybdenum dissolved therein so that the molybdenum is present in amount of from about 40 grams to 75 grams per liter of acid solution, the rate of molybdenum dissolution becomes sufficiently slow that the acid is regarded as "spent". In the usual practices of the prior art, this spent acid solution has suitably been treated and then discarded, since it has not been economical to recover the dissolved molybdenum.